The FAA rules are as follows:
Less than 20 pounds use a rear-facing CRS
From 20 to 40 pounds use a forward-facing CRS
More than 40 pounds use an airplane seat belt
A child may also use an alternative device, such as a a harness-type device, if it is approved by the FAA. The FAA has approved one device appropriate for children weighing between 22 and 44 pounds. This type of device is not safe for use in motor vehicles.
No booster seats are allowed.
Make sure your CRS is government approved and has "This restraint is certified for use in motor vehicles and aircraft" printed on it. Otherwise, you may be asked to check the CRS as baggage.
Make sure a child safety device is approved and has "FAA Approved in Accordance with 14CFR 21.305(d), Approved for Aircraft Use Only" on it.
Measure the width of your CRS. It should fit in most airplane seats if it is no wider than 16 inches.
So if your son is 40 pounds or less and you would like to use a car seat, it is fine. If he is between 40-44 pounds and you would like to use the FAA approved vest, that is fine too.
Have a safe trip!
Dallaschildren
CPS Instructor and momma to 2 sons in seats